This invention relates to an automotive navigation system, and in particular to an automotive navigation system wherein a departure point, a destination point, and the current position of a vehicle are displayed with respective marks on a display such as a cathode ray tube.
Such an automotive navigation system has been already proposed in Japanese Patent Application Laid-open No. 58-146814. This conventional system detects the running distance and the heading of a vehicle and computes the current position of the vehicle from those information. This system displays the current position of the vehicle according to the computed result on a display such as a CRT while at the same time displaying a map as picture information read out from a storage means on the display in a superposed manner, whereby a driver can determine the current position of the vehicle from the map and the mark imaged on the display.
However, since an extremely numerous amount of information is required to display the picture information forming a map, a storage means for storing such amount of information and therefore a navigation system per se must be correspondingly large-scaled and expensive. Accordingly, it is desirable to develop a small-sized and cheap navigation system suitable for boarding on a vehicle.
In a case where a departure point and a destination point are predetermined, even though a map stored in the memory is displayed on the display and a mark indicating the current position of the vehicle is displayed in such a superposed manner, the map to be displayed on a reduced scale is in certain conditions so small that the current position of a vehicle can not be clearly seen. Furthermore, if the distance between the departure point and the destination point is far so as to require a plurality of sequential maps, it is very hard and cumbersome to grasp the entire running route.
Although it is not necessarily impossible to solve these technical problems with a memory having a large capacity as well as a high speed arithmetic device, the size of the whole system becomes very large so that it is difficult to mount the same in the vehicle both physically and economically.
Also, in a case where the heading of a vehicle is measured by detecting the earth magnetism, the heading to be detected is provided with a relative angle derived from the direction of the earth magnetism and the running direction of the vehicle (vehicle heading) and therefore it often disagrees with the actual geographical heading. In other words, while the direction of the earth magnetism generally points from the south to the north, the actual geographical north often disagrees with the direction of the earth magnetism depending on which region of the earth is being considered. The angular difference between both of the above is called a declination. In Japan, for example, there are declinations of between 5-10 degrees towards the west.
Therefore, if the mark of the current position of the vehicle computed by the use of the vehicle heading detected on the basis of the direction of the earth magnetism is displayed in a superposed manner on the screen displaying a map prepared on the basis of the geographical north, there will be a considerable error between the true or actual current position and the current computed and displayed position.
In order to eliminate an error due to such a declination, a system which sets a declination value through an external operation for the region where the vehicle is presently running and which corrects, based on the set value, the running heading detected has been disclosed in, e.g. Japanese Patent Application Laid-open No. 58-5610. However, this system has a disadvantage that the declination value must be set over again whenever the region where the vehicle is running changes, causing complicated operations.
On the other hand, there have been disclosed, "Cathode-Ray Tube Information Center with Automotive Navigation" published in SAE Technical Paper Series 840485 by M. W. Jarvis and R. C. Berry, and "On-Board Computer System for Navigation, Orientation, and Route Optimization" published in SAE Technical Paper Series 840313 by P. Haeussermann. Both publications are based on an international Congress & Exposition held in Detroit, Michigan on Feb. 27-Mar. 2, 1984. In the former literature, an approximate position of a vehicle is determined from the communication with a satellite, and a more accurate position is determined and displayed on the CRT by means of a self-contained navigation using an earth magnetism sensor in the vehicle. The latter literature discloses a composite system of a route guide system in trunk (main) highways using distance information and a destination indicating system within a city using distance information and heading information.